Thermoelectric cell and reactor



y 1967 G. M. GROVER ETAL 3,321,646

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THERMOELECTRIC CELL AND REACTOR Original Filed June 18, 1959 8Sheets-Sheet -im O g o E Uranium g Carbide 25 @0' U. I m I I E o m n. Eq 0 -3 l0 Znrcomum Carbide I m I I I i000 I200 I400 I600 i800 2000 2200Emwier Tempemmre, Deqvees Kelvin WITNESSES i INVENTOR. Z George M.Graver Robert W. P/dd @45 BY Ernest [M 501ml G. M. GROVER ET AL,

THERMOELECTRIC CELL AND REACTOR Original Filed June 18, 1959 8Sheets-Sheet 5 i ,Amperes per Ceniimeie 6 5 1. I l I I I I I I I I I I lI I I I IIIIII IIIIIIII oa oz IIIIIII I I I I V/T/VESSES:

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George M. Grover Robert W. Pia'd BY Erflesf W. .Sa/m/ Audi? 8Sheets-Sheet 6 ay 23, W67 ca. M. GROVER ETAL THERMOELECTRIC CELL ANDREACTOR Original Filed June 18, 1959 y 3, 1967 G. M. GROVER ETAL3,321,646

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M George M. Grover, Robert W P/aa' BY Ernesf m .So/mi Wflai M? 3,3ZLM6Patented May 23, 1967 Free 3,321,646 THERMUELECTREC CELL AND REACTOEGeorge M. Grover, Los Alamos, N. Mex, Robert W. Pidd, La Jolla, Calif.,and Ernest W. Salmi, Los Alamos, N. More, assignors to the United Statesof America as represented by the United States Atomic Energy CommissionContinuation of application Ser. No. 821,339, June 18, 1959. Thisapplication Dec. 27, 1963, Ser. No. 337,967 38 Claims. (C1. 31{l--4) Thepresent application is a continuation of an application filed by thepresent inventors on June 18, 1959, SN. 821,339, now abandoned. Thatapplication was a continuation-in-part of an application filed by thepresent inventors on Mar. 3, 1959, SN. 796,991, and now abandoned, thelatter in turn being an undiminished continuation-in-part of anapplication filed on Oct. 1, 1958, SN. 764,731, and now abandoned.

An object of the present invention is to provide a thermionic emitterelectrode capable of continuous operation at high temperatures toprovide electrical currents far exceeding those of any prior artemitter.

It is also an object of the present invention to provide means forconverting the energy released in nuclear fission directly to electricalenergy by including fissionable nuclei as a part of the emitterelectrode in a thermoelectric ceil containing a collector electrode anda low resistance plasma of an easily ionizable gas, and utilizing suchfission energy to heat such emitter electrode to a high temperature. Byextension, the present invention includes a fission reactor in which allof the fissionable material is incorporated in such emitter electrodes,a battery of such cells being assembled to provide a critical assemblyand the outputs of such cells being combined to provide a highelectrical power output.

Every conducting medium may be characterized by its thermoelectricpower, a quantity expressed in units of potential difference per degreetemperature difference for a particular temperature difference from onepart of the medium to another. In metals, the thermoelectric powers areof the order of one microvolt per degree centigrade. These low valuesare commonly attributed to the fact that the electron gas in a metal isdegenerate, i.e., the electrons are virtually non-interacting. I11non-degencrate media, such as the electron clouds in a vacuum, in aplasma, in a semiconductor or an electrolyte, the characteristic valueof the thermoelectric power is about 1000 times as great, or more nearlya millivolt per degree centigrade.

However, in considering the possibilities of exploiting thesenon-degenerate media, semiconductors do not appear to be suitable forthermoelectric cells because of their temperature sensitivities andtheir extremely delicate compositions. At the high operatingtemperatures of the present invention (15603000 G), semiconductors losetheir room temperature stability and behave in an erratic manner. Thedelicate compositions which give them their unusual properties,including very small amounts of impurities, would be easily upset byinteraction with high level radiation 'riuxes, such as the neutron andgamma-ray fluxes of a fission reactor.

It has also been found that both vacua and semiconductors have quitehigh internal impedanccs in comparison with those of the plasmasutilized in the various embodiments of the present invention. Forinstance, when a vacuum diode is operated without an applied voltage,but with one plate at a considerably higher temperature than the other,a current of the order of one microampere per square centimeter ofemitting surface will flow through an external resistor connectedbetween the two plates for an electrode spacing of the order of onecentimeter. This type of thermoelectric conversion depends on electronemission from the heated surface and electron absorption on the coldersurface, and the current flow therein is limited by the space chargecreated by the electron cloud in the interelectrode space.

It has been found that the addition of a very small amount of an easilyionizable gas such as cesium vapor in the diode envelope changes itscharacteristics in a little effect on the EMF developed between theelectrodes, but the impedance of the diode is lower by many orders ofmagnitude for the same geometry and for the same conditions oftemperature and external resistance. The net result is a many foldincrease in current flow, of the order of 10 and a correspondingincrease in the power delivered to the external load.

Irving Langmuir, K. H. Kingdon and others investigated some of theeffects of a cesium plasma in a diode in the 1920s and 1930s andrealized that such a plasma, like that of many gases, is very effectivein avoiding the space charge effect of a vacuum diode. They also foundthat a low pressure cesium vapor enhances the current emission ofcertain thermionic cathode materials with the latter at a temperaturenot exceeding about 1G00 K., but apparently they did not realize thatsuch properties could be exploited to make such a diode useful as asource of electrical power, i.e., that high output currents could beobtained without the use of an external voltage source. They werepreoccupied with the various phenomena observed at the emitter surfaceand with the positive ion currents obtained between a tungsten emitterand a surrounding collector electrode with the tungsten above a certaincritical temperature (1150-1200 K), a cesium vapor at essentially roomtemperature and a negative voltage applied to the collector, pointingout that such a device so operated is highly useful and accurate inmeasuring the vapor pressure of a gas. See, for example, Science, 57, 58(1923) and Phys. Rev., 51, 753 (1937).

One of the outstanding characteristics of such an ionized vapor, orplasma, is its disorder, or randomness of direction of its constituentparticles. Collisions with neutrons or other radiations will have littleeffect other than to increase the average particle energy and degree ofionization, effects which are wholly salutary. In addition, the easilyionized materials suitable for such plasmas have insignificantmacroscopic absorption cross sections for neutrons because of the lowplasma density. all of. which make thermoelectric cells utilizing suchvapors eminently suitable for use in extracting the thermal energyliberated in a nuclear fission reactor.

The present invention and the results produced thereby can be moreclearly understood by reference to the attached drawings, herebyincorporated herein by reference, in which:

FIGURE 1 is an elevation cross section of an embodiment of a basicthermionic generator utilizing flat plate geometry,

FIGURE 2 is a graph of the power output obtained with the FIGURE 1embodiment as a function of the external resistance for varioustemperatures of the heated plate,

FIGURE 3 is an embodiment of a basic thermionic generator utilizingconcentric cylinder geometry,

FIGURE 4 depicts short circuit current-emitter temperaturecharacteristics for the FIGURE 1 embodiment utilizing various emittermaterials and also includes saturated emission currents for suchmaterials in vacuo,

FIGURE 5 contains zero field saturated emission current curves foruranium carbide and zirconium carbide emitters,

FIGURE 6 is a similar curve for an emitter of 80 atomic percentzirconium carbide and 20 atomic percent uranium carbide,

FIGURE 7 is a sectional view of a fission reactor fuel elementconsisting of a multiplicity of thermoelectric cells of the presentinvention,

'FIGURE 8 is a sectional view of a single thermoelectric cell assembledwith a dewar flask for testing in a fission reactor, and

FIGURE 9 are the open circuit voltage and short circuit currents of theFIGURE 8 embodiment as functions of the maximum power of the reactor inwhich the cell was tested.

In the embodiment of FIGURE 1 described by George M. Grover (US. Patentapplications S.N. 249,962 filed January 7, 1963), the principal elementsare the heated plate 1, hereinafter referred to as the emitter, thevapor 2 of an easily ionizable gas, and the cold plate 3, hereinafterreferred to as the collector. Collector 3 is mounted within a gastightcontainer 5 having a filling and pump-out lead 6 connected thereto andsealed at its open end by closure 4. Emitter 1 is mounted on the insideof closure 4, which is preferably metallic and of small thickness forgood transmission of heat. Heat is supplied to closure 4 through meansgenerally indicated as a tubular container 7 mounted thereon.

The entire assembly of gastight container 5 and heat supplying container7 is mounted within a vessel 8 suitable for containing a hot insulatingfluid 9, e.g., a silicone oil. A portion of this oil is continuouslycirculated by pump 10 through pipes 11, 12, 13 and 14 to the coil 15 onthe underside of collector 3 to conduct away the thermal energy of thelatter received by radiation from the emitter 1.

In the arrangement illustrated, parts 7, 4 and 5 are metallic forconvenience in making connections to the external resistor R. Aspictured, resistor R is connected between the emitter 1 and thecollector 3 through the ammeter A, voltmeter V being used to measure thevoltage developed across the resistor R. The emitter 1 is approximately2 square centimeters in area and is spaced about 1 centimeter from thecollector 3. Also shown are the thermocouple 18, used to measure thetemperature of collector 3, and differential thermocouple 19, used tomeasure the temperature difference of the coil used to cool collector 3.Knowing this drop and the rate of flow, the heat extracted fromcollector 3 can be calculated, and from this and the known input toemitter 1, the efficiency of the device can be determined. Insulatingbushing is used for electrical separation of container 5 and coolantflow lines 13 and 14.

In preparing the cell for operation, the liquid form 16 of an easilyionized vapor is introduced through lead 6 under an oxygen-free,water-free, atmosphere. The container 5 is then evacuated to a hardvacuum (10 millimeter Hg) and is permanently sealed. The liquid 16 isthen partially vaporized by heating the bath liquid to the appropriatetemperature for the cesium vapor used. To obtain the results shown inFIGURE 2, vapor pressures of 10- mm. Hg to 1 mm. Hg can be obtained bythermostatically controlling the temperature of the silicone oil bath attemperatures up to 300 C. through heater 17. Oxygen and water must berigorously excluded because of their great affinity for cesium and theother alkali metals, which may be substituted for cesium. While minoramounts of inert gases such as argon may be tolerated, any such gas hasits own impedance as a cell element, an impedance which acts in serieswith that of the cesium to depress the current flowing in its absence.

The results shown in FIGURE 2 indicate those obtained with a bathtemperature of 250 C., under which conditions the collector temperaturewas about 300 C. They also indicate a maximum power output of about 20watts, or about 10 watts per square centimeter of emitter surface. Themaximum short circuit current observed was 40 amperes.

No specific means for heating emitter 1 are shown in FIGURE 1, as suchmeans may assume a wide variety of forms. Hot flue gases may beutilized, heat liberated by chemical combustion within container 7 maybe employed, or according to this invention, energetic particles such asalpha particles or fission products formed in a reactor may be directedto impinge on closure 4. Where more penetrating radiations such as betaparticles, gamma-rays, X-rays or neutrons are to be absorbed, it will ofcourse be necessary to add a suitable thickness of absorptive materialabove closure 4. To utilize the heat liberated in a fission reactor, theemitter may be made in the form of a massive element of a refractorymaterial which includes the fission fuel, e.g., a rod of a solid soluionof zirconium carbide and uranium-235 carbide, with a surroundingcollector sleeve providing a gap for the easily ionized vapor, thesuba-ssembly forming a fuel cell. When a group of such cells isassembled in a neutronically critical configuration, the emitter will beself-heated in absorbing the kinetic energy of the fission products andthe electrical outputs of the cells may be combined through suitablecircuitry and utilized outside the reactor. Where necessary, thecollector elec trode temperature may be maintained at a low value bysuitable gas cooling or other conventional cooling means.

Further, in accordance with the invention, the emitter electrode may bemade self-heating by fabricating the same to consist in large part ofintensely radioactive materials, in particular the intense alpha andbeta emitting nuclides. Many such nuclides are available as fissionreactor waste products which are easily convertible to refractorycompounds such as oxides or carbides. Examples of such nuclides are Po,P and Sr, and it should not be particularly difiicult to obtain thesematerials in concentrated forms having readiation strengths of 10 to 10curies. It can be shown from the work of the present inventors that suchself-heated cells atf-ord power outputs of the order of one kilowatt perpound of necessary structure.

The embodiment of FIGURE 1 illustrates the type of apparatus used inlaboratory investigations of the basic thermionic generator described inS.N. 249,962 and as such contains many features unnecessary tocommercially practical embodiments and is unnecessarily limiting incapacity. FIGURE 3 illustrates a practical embodiment of the basicthermoionic generator described in Grover S.N. 249,962 stripped to bareessentials and peculiarly adapted to utilize hot gases.

The embodiment of FIGURE 3 consists essentially of a pair of concentricconductive cylinders .31 and 32 held in place by insulating end rings 37and 38 .to define an annular chamber 33. Chamber 33 is exhausted thoughlead 35 and liquid 34 is admitted thereto by the same means. Emittercylinder 31 is connected to an external electrical circuit throughconductor 39, which collector cylinder 32 is thus connected throughconductor 40. Emitter 31 is heated by the hot gas entering bore 36 atthe lower end 41 for he direction of flow indicated by the arrow.

While bore 36 is illustrated as unrestricted from one end of the deviceto the other, it is apparent that bafiies, etc., may be provided torestrict the flow for greater heat extraction. The various possibleexpedients for heat transfer from the hot gas to the emitter cylinder 31are well known to heating engineers and hence need not be elaboratedherein.

No means is provided for cooling or heating the collector electrode 32of FIGURE 3 because it may be exposed to room temperatures or lower andthereby kept at the minimum necessary temperature by radiation from theemitter 31 and re-radiation to the outside. If nec' essary, theconventional, thermostatically controlled heat= ing or cooling means maybe added. Output efiiciency appears to depend primarily on maximizingthe temperature differential between the emitter and the collector, soall means to raise the former and keep the latter at the minimumnecessary for vaporization of the conductive medium between electrodeswill improve the operation of the device-unless increasing thetemperature of the collector decreases the plasma resistance, asdiscussed below.

In preparing the FIGURE 3 embodiment for operation, the amount of cesiumor other conducting medium necessary for the desired operating pressure,e.g., sodium, potassium or rubidium, may be added through lead 35 andthe latter sealed off. When the temperature corresponding to suchoperating pressure is attained, all of the liquid will be vaporized andhence the particle density n of the'vapor will be fixed for 'this andall higher temperatures. Such condition may be desirable where it isexpedient to limit the operating pressure, although the experimentalwork of the present inventors indicates that their thermoelectric cellsare not particularly pressure sensitive. Experiments with fixed emittertemperatures showed a fiat power output curve as the pressure was variedover the range (ll-2.0 mm. Hg, with no indication of a tendency to riseor fall at either extreme. Greater pressures are difiicult to obtainwith the experimental embodiment of FIGURE 1, as the required bathtemperatures make gasketing very difiicult.

Flat power output for the various cesium pressures in the 0.1-2.0 mm. Hgrange was obtained at a relatively large electrode spacing, 1 cm. ormore, when a tantalum emitter was used. As indicated below in thediscussion of the FIGURE 4 results, orderly increases in power andcurrent with increasing pressure are observed with a tantalum emitterand an electrode spacing of /4-il1Ci1 (0.635 cm.). Similar increases areobserved at all smaller spacings with a tantalum emitter and smiliarincreases are observed with carbide emitters at small spacings from thecollector, of the order of 0.1 cm. In such instances of pressuredependence, current and power output each varies approximately as thesquare root of the cesium pressure. It has also been observed that atquite lar e spacings, greater than one centimeter, cesium pressuresgreater than 1 mm. Hg cause a decrease in power output. This phenomenonis probably the result of insufficient ionization and/or an increase inthe number of collisions between electrons and neutral gas particles.

It is to be noted that no separate means for ionizing the conductingvapor are provided in the illustrated embodiments, although they may beprovided if desired. While the mechanisms of ionization and conductionare not fully understood and no theory has been developed which fullyexplains the behavior of the present inventors thermoelectric cells, itseems apparent that the primary ionization mechanism at the onset issimply thermal. In other words, neutral atoms in the vicinity of the hotemitter become heated and dissociate simply because the valenceelectrons are too energic to remain bound. After the onset ofconduction, 1 R heating of the plasma probably accomplishes some furtherionization. Other processes which may be involved are ionization by thephotons streaming from the hot emitter and ionization by contact betweenneutral atoms and the hot emitter. When the work function of the emitteris greater than the ionization potential of the gas, it is known thatessentially all of the gas atoms contacting the emitter are ionized;this etiect falls off rapidly as the work function of the emitterdescreases below the ionization potential of the gas. The intenseemission of electrons by the emitter probably tends to causerecombination into neutrals in the neightborhood of the emitter.

Of course, it is necessary that the emitter and collector be fabricatedof materials capable of withstanding the temperatures at which they areoperated. The results indicated in FIGURE 2 were obtained with anemitter of tantalum, a copper collector and a cesium vapor. Equally goodresults are obtainable with cesium vapor and with both electrodes madeof tantalum or tungsten under conditions otherwise the same. Anyrefractory and electrically conductive material, including alloys andsuch compounds as oxides and carbides, appears to be useable. Thecollector, of course, may have a considerably lower melting point thanthe emitter.

The present inventors have discovered that the emitter material is ofconsiderable importance in increasing the current flow and thus thepower delivered to the load under conditions otherwise identical. Theyhave discovered that the carbides of uranium and zirconium, UC and ZrC,and solid solutions thereof, have properties which make themoutstandingly superior thermionic emitters, especially when suchcarbides are used in the fused form, either as coatings on a base metalor as structural elements in themselves. Thus the power outputsindicated in FIGURE 2 and discussed above (obtained by the previouslyknown embodiment of FIGURE 1) are increased many fold for conditionsotherwise the same except that the purely tantalum emitter was replacedwith an emitter consisting of tantalum coated with a fused polycarbideconsisting of atomic percent (a/o) zirconium carbide (ZrC) and 20 a/ouranium carbide to a thickness of -inch, prepared in a manner more fullydetailed below. This emitter and the others indicated in FIGURE 4 weretested in the apparatus of FIGURE 1 with no external resistance otherthan that of the connecting cables, with a cesium pressure of 0.5 mm.Hg, and with a 0.635 cm. gap between electrodes. The temperature of theemitter was varied to yield the short circuit current vs. temperaturecharacteristic indicated in FIGURE 4, together with similarcharacteristics obtained from identical tests with a tantalum emitterand a number of cesium pressures and also with a tantalum emitter coatedwith only zirconium carbide. Also indicated in FIGURE 4 by the dashedcurves are saturated emission currents for these emitter materials in avacuum of about 10 mm. Hg for the same 0.635 cm. spacing. The values ofshort circuit current for the latter curves were determinedexperimentally for currents up to about 1 ampere and extrapolatedthereafter.

FIGURE 4 illustrates the tremendous advantages possessed by theembodiment of this invention over those of the basic thermionicgenerator as described by George M. Grover, US. application S.N.249,962. The basic thermionic generator concept is illustrated at FIGURE1 and 3 of the present application. As can be seen from FIGURE 4, thecurrents obtainable with the carbide emitter are considerably largerthan with a tantalum emitter. Thus the largest short circuit currentobtained with the tantalum emitter was 40 amperes at a temperature of2630 K., while at the same temperature a short circuit current of 124amperes was obtained with the carbide emitter. The corresponding currentdensities are 20 amperes per cm. and 6 2 amperes per cm. of emittersurface. Note that the largest current obtained with the tantalumemitter was at the highest cesium pressure reported, 2 mm. Hg.Subsequent investigations of cesium pressures as high as 6 mm. Hg.indicate that still higher currents and power outputs are obtained underconditions otherwise the same. As can be seen from the group of curvesfor tantalum currents at various pressures, the electron emissionproperty of this metal exhibits a systematic increase for increasingcesium pressure, the current (and power output) varying aproximately asthe square root of the cesium pressure. A similar pressure dependencewas exhibited by the carbide emitter at electrode spacings of the orderof 0.1 cm. The same dependence may exist for carbide emitters andsomewhat greater electrode spacings, but the experimental work thus fardoes not allow of any definite statement.

The carbide emission curves of FIGURE 4 for the cases where the emitteris used with a cesium plasma deviate to the right from the correspondinguse in a vacuum. This indicates that such currents are plasma limited, acondition somewhat analogous to that of a space charge limited current.In the case of metallic emitters, the benefit of the plasma is not fullyexploited because of the electron emission limit of the metal, whilewith the carbide emitters, the larger electron emissions permit a fullerutilization of the benefits inherent in the plasma. Indeed, there aresome experimental points at which maximum utilization obtains andfurther increases in current must await decreases in plasma impedance,e.g., the aforementioned plasma limited regions of thecurrent-temperature characteristics of the carbide emitters of FIGURE 4.

The above indicated superior electron emission characteristics of thecarbides also led the present inventors to the conclusion that suchmaterials might be used to good advantage in ordinary vacuum tubes andgas filled tubes employing thermionic cathodes. Accordingly, emitterswere prepared coated with uranium carbide, zirconium carbide, and asolid solution consisting of 80 atomic percent ZrC and 20 atomic percentUC. Emitters of such materials were prepared for evaluation by aremelting the carbides under an argon atmosphere and allowing them to fuseon /s-inch diameter by /s-inch thick tantalum disks in a -inch coatingthickness. This technique also required the preliminary formation of acoating of tantalum carbide on the tantalum base by outgassing thetantalum surface at 2200 C. in vacuum and heating the surface in contactwith outgassed graphite powder, in a vacuum or inert atmosphere, at 2000to 2200 C. for a period of about 15 minutes, as indicated in US. PatentNo. 3,020,632 to Kriko-rian et a1. Tantalum was selected for the backingbecause it is quite stable at high temperatures, but other conductivematerials having high temperature stability can be used in its stead,e.g., tungsten. It is also possible to use the UC-ZrC polycarbidewithout a backing, as in the form of rods.

Each carbide was in itself prepared previously by arc heating thepurified elements together in a previously gettered argon atmosphere.The polycarbide solid solutions were prepared both by hot pressing ofthe two carbides in powder form (-l40 mesh) with 5 weight percent cobalt(-325 mesh) in graphite dies at 3000 p.s.i. and 1900" C., laterdistilling out the cobalt at 2000 C. in vacuo, and by inductivelyheating the blended carbide powders with no cobalt at 2000 C. for fourhours in a graphite crucible. The latter technique resulted in asintered rod which could be handled without breakage. The carbides had asilvery, dense, metallic surface, exhibiting no voids but havingmacroscopic crystalline patterns.

The electron emission characteristics of the various carbides were thentested in diode valves, using the coated plates as prepared above ascathodes. In each diode the cathode also served as a closure, the metalside facing outward and being heated by electron bombardment from anelectron gun. Opposite the inwardly facing carbide surface and spacedtherefrom at 0.635 cm. was a metallic disk collector or plate electrodeof the same diameter as the cathode and a surrounding coplanar guardelectrode to provide a uniform electric field for electron acecleration.Both electrodes were mounted in an envelope evacuated to 10- mm. Hg. Theguard ring was grounded directly and the collector was grounded throughan ammeter. The potential of the emitter was varied with respect to thegrounded electrodes up to 3 kilovolts to obtain saturated emissioncurrents and to obtain the Shottky correction to the zero-fieldcurrents. All currents discussed below are thus corrected byextrapolating a plot of log i vs. V to zero volts, yielding the zerofield saturated current density i The results thus obtained arepresented in FIGURES and 6 of the drawings, FIGURE 5 showing theseparate characteristics of UC and ZrC, while FIGURE 6 presents 8 thecharacteristic of a solid solution of the two carbides consisting ofatomic percent (a/o) ZrC and 20 a/o UC. Note that the lattercharacteristic is essentially identical with that of UC, the polycarbidehaving the higher electron emissivity of the UC at any given temperatureand the greater ductility of the ZrC. UC alone tends to crack onrepeated thermal cycling, but no such behavior is manifested by the ZrCor polycarbides employed, even when rapidly and repeatedly cycled fromroom temperature to 2400 C. and back.

The data disclosed in FIGURES 5 and 6 can be fitted to the Richardsonemission curve where T=absolute temperature of emitter surface,

Kelvin q electron charge, coulombs k=Boltzman constant, ergs/ degreeKelvin =work function of emitter surface at temperature T,

volts A=a semi-empirical constant,

degrees amperes Emitter WE Vo t cm.--K.

To compare the values of i in FIGURES 5 and 6 with those of a cleanmetal, note, e.g., that the polycarbide current density is 0.16 amp/cm.at l800 C. The corresponding current density of tantalum at thistemperature is 10- amp/ch1 indicated a l600-fold increase in going tothe carbide.

The =UC and UC-ZrC emitters discussed above also have other propertieswhich make them peculiarly suitable as thermionic coatings orstructures. No activation schedule, such as the flashing of thoriatedtungsten or the gradual increase in current of oxide coatings, isrequired, nor is it necessary to take special steps to clean the surfaceof the present carbide emitters prior to use. Exposure to ambientatmospheres has no effect even though lasting for days, and the presentemitters may be re-used in vacuum systems without the delays necessaryto reactivate the thermionic cathodes of the prior art. This particularadvantage makes the present emitters ideal for use in apparatus whichmust be periodically opened for one reason or another, e.g., betatrons,cyclotrons and other charged particle accelerators.

The results obtained with the carbide emitters depicted in FIGURES 4, 5and 6 are absolutely reproducible, i.e., there is no gradual loss ofemissivity in repeated operation. Such results are also obtained withoutthe time lag peculiar to many of the good emitters of the prior art, inwhich several seconds may elapse between energizing the cathode heaterand the appearance of maximum current.

Probably the most outstanding advantage of the carbide emitters hereindisclosed is the ability to operate at much higher current densities andtemperatures than the thermionic emitters of the prior art, especiallythe thoriated tungsten cathodes. The latter have an upper operatinglimit of about 1 ampere per square centimeter, Whereas the emitters ofthe present invention have already been operated without ill effects atcurrent densities as high as 60 amp./cni. as indicated in FIGURE 4, andextrapolated values as high as 1000 amp/cm. appears feasible. When athoriated tungsten filament is operated above its limit, evenmomentarily, the thoria appears to boil off and the enhanced emissionproperties are completely lost until the filament is re-activated.Emission from the carbide materials, on the other hand, appear to belimited only by the temperature at which such material is structurallystable, about 2900 C. for the specific polycarbide disclosed. (UC meltsat 245B C., ZrC at 3500 C.)

The use of the carbides disclosed herein is not limited to the physicalforms of coatings or inclusion in a metal, as are the oxide emitters ofthe prior art. This follows because such carbides approach the metals inboth thermal and electrical conductivity, unlike the highly insulatingand highly resistive oxides of the prior art.

While details of only a limited number of carbide compositions aredisclosed herein, and only coatings on metallic bases and unitarycarbide structures have been discussed, there are reasons to believethat many other compositions and combinations will yield comparableresults. Thus when UC carbide was melted in close proximity to a highlyincandescent tungsten filament in a vacuum diode, the electron emissionfrom the filament was observed to increase by a factor of 3. A smallamount of work with niobium carbide indicates that its electron emissioncharacteristics may approach that of uranium carbide. Other carbideswhich may lie in the same range are those of thorium, protactinium,plutonium, neptunium, and americium. The other carbides of uranium areprobably equally as good as the monocarbide dealt with. There is goodreason to believe that many other proportions of UC and ZrC will producecomparable or better results, as their crystal structures areisomorphous and they appear to be completely miscible.

The current densities disclosed above for uranium carbide and uraniumcarbide diluted with zirconium carbide were obtained with UC preparedfrom the non-fissionable isotope U (at least non-fissionable except byhighly energetic neutrons). The emission characteristics do not dependon any of the nuclear cross sections of uranium, and the presentinvention is thus believed to be fundamental is exploiting the elementin a manner which does not depend on its nuclear characteristics.However, the highly fissionable isotopes such as U may be used withoutany immediate effects on emissivity, as the rate of electron emissiondoes not depend on the composition of the nucleus.

If the UC in an emitter is made of uranium enriched in U to provide acritical assembly, either in one cell or a number of cells disposedadjacent one another, the fission products may not be held in theemitter structure at the high temperature of the latter. Many of themare gases which could enter the plasma and gradually destroy its utilityas a low resistance, non-degenerate electron medium.

Such behavior is in direct contrast to the activity taking place in aconventional heterogeneous fission reactor, especially those using soliduranium or plutonium fuel elements. In such elements the fissionproducts remain in the fuel and there is no way of eliminating themexcept except by withdrawing the element and processing it to remove thefission products. The unfortunate aspect of such a situation is that theprocessing must be accomplished before any appreciable part of thefissionable element content has been used, as the economic time forprocessing is dictated by the rate of accumulation of fission productpoisons, i.e., those which capture neutrons prodigally. In thecomparable elements of the present invention, the high temperature fuelelement emitters tend to cleanse themselves of fission products and neednever be processed except when it is desirable to replenish thefissionable nuclide material. All that is necessary to preserve the goodcharacteristics of the plasma is a continuous processing of the gas toremove the fission products therefrom. Such processing is much easier toaccomplish with a mobile medium such as a gas than with a solidmaterial.

To demonstrate the advantages of the thermoelectric cells of the presentinvention using carbide emitters with uranium enriched in the U-235isotope, such a cell was prepared and tested in an operating nuclearfission reactor. FIGURE 8 is a longitudinal section of this cell, shownhere in full scale. The cylindrical fuel pin 101 was fabricated by hotpressing equal weights of uranium carbide (UC) and zirconium carbide(ZrC) into a solid rod about .4-inch in diameter, the fuel pin likewisebeing pressed onto the hollow tantalum support 102, this assembly thenbeing sintered, machined to a final diameter of 0.71 cm. and threaded atthe tantalum end. While no demarcation in fuel pin 101 is indicated inFIGURE 8, only the lower half contained uranium carbide in which theuranium was enriched with the U isotope (94%), the upper half containingonly natural uranium. This was done to insure a thermal gradient fromthe lower end of fuel pin 101 to the copper base 103 into which thetantalum support 102 was threaded. The lower half of fuel pin 101 thushad a circumferential area of 2.84 (2111. an end area of 0.39 cm. avolume of 0.50 cm. and a total mass of 4.5 grams, of which about 1 to 2grams were U The balance of the thermoelectric cell of FIGURE 8consisted of the cylindrical stainless steel collector 164, to which wasaillxed a spiral copper cooling fin 105, the latter serving both todissipate heat received by the collector to inner wall 107 of metaldewar cylinder 108 and the circulating oil coolant 106 and to secure thefuel Within the dewar. The radial gap between electrodes was about 0.7cm. The cell proper was closed by an annular alumina insulator 111 andnickel flange members 112 and 113, flange 112 being sealingly secured toboth the insulator Ill and the collector 194, and flange 113 to theemitter base 193 and to insulator 113. The central opening 114 extendingfrom the top of emitter base 103 was provided to accommodate athermocouple tube 115. Above the structure shown were a closure for thedewar flask, a long supporting tube for the entire structure, electricalleads secured to emitter base 163 and inner dewar wall 107, and an oilreturn lead.

These leads were connected to a remotely operated switch installed justabove the cell, such switch having positions to measure open circuitvoltage, short circuit current and maximum power output. Coolant oil 106was supplied through oil lead 116 to control the temperature ofcollector 104 and the cesium pressure within the cell. Perforated crossmember 117 served as a radiation shield for the cesium pool. Helium atatmospheric pressure was used between the inner wall 107 and the outerWall 109 of dewar tlask 108 as a means of conducting gamma heat frominner wall 107. Use of the cell within such a dewar flask made itpossible to immerse the assembly in the cool (37 C.) water of thereactor without afiiecting the temperatures Within the cell. Theelectrical path from the collector was through both the spiral coolingfin and a heavy strap (not shown) to inner dewar Wall 1127.

Prior to assembly, the cell was evacuated through lead 120, cesium wasadmitted through the same lead, and the lead was sealed off. Thethermoelectric cell and dewar assembly were secured to theaforementioned long tube with the remotely controlled switch in placeabove the dewar, and the oil and instrumentation leads were extendedthrough to the upper end of the tube. The entire assembly was thenlowered into one of the central empty fuel element holes in the LosAlamos Omega West reactor, a heterogeneous water cooled and moderatedmaterials testing type of reactor using solid enriched uranium fuel rodsjacketed with aluminum. This particassembly.

asaneae ll ular reactor has an approved maximum power capacity of aboutmegawatts (thermal).

The reactor was brought up to maximum power by customary means(manipulation of control rods) in about half megawatt steps, andoperated at such power for about five hours. Short circuit current wasmonitored continuously, and open circuit voltage was determined forfractions of maximum power and occasionally after reaching full power,yielding the results indicated in FIGURE 9. The open circuit voltageremained essentially constant during the five hour period, but the shortcircuit current dropped off about 20% from the 37-ampe-re valueindicated during the first hour of operation, remaining at about 30amperes thereafter. A malfunction of the switch in its third positionprevented any measurement of maximum power output. During this periodthe averag temperature of the coolant oil contacting the collectorelectrode was about 260 C., from which it is known that the cesiumpressure in the cell was about 0.1 mm. Hg.

At the end of this first five hour period, the reactor was shut down fora weekend (-60 hr.) with the thermoelectric cell assembly in place. Itwas then brought up to maximum power and operated at this level for asecond five-hour period. The open circuit voltage and short circuitcurrent were the same as before the first shutdown and exhibited littlevariation during the second five hours of operation. Again maximum poweroutput could not be measured because of the faculty switch. The poweroutput on short circuit could not be calculated because the resistanceof the external path was not known.

At the end of the second five hour period, the cell was stillfunctioning as indicated, but it was decided that the cell should beremoved for analysis. This analysis indicated that the fuel pin had beensubject to an average thermal neutron fiux of neutrons per cm. per sec.and had experienced 6 X 10 fissions.

To determine the emitter temperature during the inpile test and themaximum power output obtainable during such test, a cell of .a similargeometry and the same spacing and cesium pressure was operated byheating the emitter electrically. By correlating the open circuitvoltage and short circuit current data obtained with this cell asfunctions of temperature with the data obtained during the in-pile test,it is estimated that the emitter temperature during maximum reactorpower operation was 2000 C. From this information and from the knownpower output of the electrically heated cell as a function of emittertemperature, the maximum electrical power output obtainable during thein-pile test would have been 30 watts.

The above results are significant for the following reason. Theydemonstrate that the thermoelectric cells of the present invention canbe utilized to obtain electrical energy directly from the energyreleased in fission. It is noted that the high temperature of theemitter during the in-pile test could not have been achieved except byabsorption by the emitter structure of the energy released by fission ofits own fuel atoms, principally the kinetic energy of the fissionfragments. It can therefore be seen that ionization of the easilyionized gas is caused principally by the kinetic energy of the fissionfragments, directly or indirectly. Any energy released by fission in theadjacent reactor fuel rods has only a negligible effect, as the fissionfragments produced therein could not have penetrated to thethermoelectric cell, and other energetic particles or radiations have atthe most a negligible heating effect on the cell (most of the neutronsare moderated to thermal energies, alphas are stopped, most of thegammas pass on through, and betas are negligible).

FIGURE 7 illustrates a fuel rod composed of a multiplicity of thethermoelectric cells of the present invention disposed in voltageadditive relationship. A suitable number of such rods may be disposed ina low temperature coolant such as room temperature water to form acritical This fuel rod consists of fuel rod base 44 and shell 52 weldedthereto as a container, both, e.g., of aluminum and serving together asthe electrical path from connector 85 at the top to the emitterelectrode of the bottom cell. Inside the shell are the bottom cell, aplurality of middle cells, a top cell and the fuel rod cap, all suchcells up to about the middle of the top cell being surrounded by acontinuous insulating sleeve 58, the latter being of slightly smalleroutside diameter than the bore of shell 52 to define the annular passage69 for the return of condensed cesium from the top of the fuel rod towell 61. Threaded to reduced diameter portion 46 of base 44 is the base47 of the first emitter electrode, e.g., of tantalum or zirconium. Thisbase is butted tightly against shoulder of fuel rod base 44 and has anoutside diameter less than the inside diameter of shell 52 to provideannular gap 61, the latter serving as a well for liquid cesium 51.Insulating cylinder 58 rests against shoulder 48 of emitter base 47 andis provided with a number of radial slots 62 extending across the bottomas vapor passages. Surrounding emitter base 47 inside the bore of sleeve58 is a second insulating sleeve 56, of smaller diameter than the firstsleeve 58 and extending longitudinally only over a part of the emitterstructure of the bottom cell. Insulating sleeve 56 has a smaller insidediameter than the diameter of the emitter base 47 to provide a gaspassage and interelectrode gap 64 around the cylindrical, center mountedemitter structure and is also provided with a number of bottom radialslots 63 to provide gas passages between gas passages 62 and 64.

The balance of the emitter structure of the bottom cell consists oftransition section 54, composed of ZrC, and fuel section 55, composed ofa solid solution of ZrC and UC. Each of these sections is fused to theadjacent section or sections and has the same diameter as stud portionof emitter base 47. Surrounding the emitter and spaced therefrom by theannular gap 64 is the cylindrical collector electrode 53 composed, e.g.,of zirconium, tantalum, copper or stainless steel. Collector 53 buttsagainst the top of insulating sleeve 56, is threaded or serrated on itsinside surface 57 to provide a maximum surface and thus reduce theimpedance of the cell, and extends into the first middle cell in areduced diameter portion to form the base of the emitter electrode ofthe first middle cell. It is to be noted that collector electrode 53 hasa slightly smaller outside diameter than the inside diameter ofinsulating sleeve 58 to define a small annular passage 76 extending fromthe top of insulating sleeve 56 to the bottom of the correspondinginsulating sleeve of the first middle cell. This gap permits readyassembly of the cell and furnishes the space required for expansion ofcollector 53. The threaded inside surface 57 of the collector electrodeterminates in smooth surface -65 to provide thread relief. Thereafter,proceeding to the top of the collector, the central passage includesfrusto-conical cavity 66, longitudinal center cavity '67, and radialpassages 68, the latter terminating in annular passage '71 between thereduced portion 60 of the collector and the short insulating sleeve 59of the first middle cell.

The division between cells as indicated in FIGURE 7 is somewhatarbitrary, as the collector electrode of each lower cell also serves asthe emitter electrode of the adjacent upper cell, and the entire fuelrod is provided with a continuous vapor passage between successivecells. Except for the fact that the bottom cell is provided with asomewhat larger emitter base, each of the middle cells is essentiallyidentical with the bottom cell, transition sections and active fuelsections of the emitter electrodes being identical, the collectors beingidentical and the gaps between paired electrodes being identical.

Proceeding to the top cell, the emitter structure and the shortinsulating sleeve thereof are identical with those of the middle cells.The differences lie in the fact that the central gas passage terminatesin a blind end 73 and downwardly extending radial passages 74 areprovided through collector '78, and insulating sleeve 58 terminatesshort of such radial passages 74. The purpose of these 'tinuous use atelevated temperatures.

13 modifications is to provide a cold region surrounding the top of thetop cell for the condensation of the cesium vapor and its return to thebottom of the rod through annular passage 69. In addition annular gap 72is provided at the base of collector 78 for axial expansion of the arrayof cells.

Stud portion 80 of top collector 78 is threaded to metal rod 81 tocomplete the electrical circuit through connector 86. This rod 81 isprovided with central passage 77 and downwardly extending radialpassages '76 to the vapor space 75 between shell 52 and both rod 31 andcollector 78, thereby providing means for vacuuming the fuel rod priorto operation and for continuous withdrawal of the vapors in space 75.Since most of the cesium will be condensed on the inside of shell 52,the bulk of any vapor withdrawn will consist of gaseousfission'products,

which may be withdrawn through 7'7 and disposed of outside the reactor.

The fuel rod is sealed at the top by annular insulating rings 82, e.g.,of alumina, and by annular springs 83 and 84, the latter being welded toboth the insulating rings 82 and aluminum shell 52. It has not beenfound feasible to employ the usual type of closure, e.g., a flat capsecured to shell 52 with a central passage and gasket to accommodate rod51, as no gaskcting material has been found which will withstand theradiation fiux to which it is exposed in a fission reactor. Central rod81 and shell 52 may be terminated as convenient for electricalconnections.

To insure positive thermal contacts between collectors 53 and insulatingsleeve 58, and also between insulating sleeve 58 and shell 52, multiplegaps 70 and single gap 69 are provided with corrugated beryllium-copperspacers, indicated in FIGURE 7 as 87 and 88, respectively, there beingone spacer 87 for each gap 79. This type of element also allows of easyassembly and, in the gap 69, furnishes the necessary space for the flowof cesium to well 61.

BeO was chosen as the material for insulating sleeve 58 and insulatingsleeves 56 because it is shock resistant and has the proper thermalconductivity. Since collector electrodes 53 will be operated atessentially 300 C. and the shell 52 will assume the approximately C.temperature of the aqueous coolant, the BeO sleeve will con- .tainessentially the full 300 C. gradient. Beryllia is alsocompatiblenuclearly,as it has a low absorption cross section forneutrons of all energies, and its low atomic weight makes it a goodmoderator. Other suitable insulating materials such as A1 0 may besubstituted.

Because the electrical conductivity of the coolant is likely to beuncertain, the outside of shell 52 and fuel rod base 44, and 81, 86 and85 should be provided with an insulating film such as an oxide ofzirconium or aluminum, e.g., by applying a coating by flame deposition.

Cesium was selected for the above mentioned experiments both for theease with which it may be volatilized and because it has the lowestknown ionization potential of all elements, namely 3.88 volts. Otheralkali metals such as K, Rb and Na may also be used, as they have lowvapor pressures and low ionization potentials, but the fact that thelatter are somewhat higher than the ionization potential of cesium meansthat a somewhat .higher temperature is required for the same degreeionization.

A particular advantage possessed by embodiments of the present inventionis that there is no consumption of parts in the operation thereof, otherthan fissioning of l the uranium content, if this is the method chosenfor heat generation. The cesium vapor is not consumed, and theelectrodes are of sturdy material, with no brittle coatings, and do notbecome subject to shock breakage by con- The only thing consumed inoperation is the heat extracted from the particular heat source used,heat which otherwise would be wasted but is converted into usefulelectrical energy by the methods and apparatus of the present invention.

As may be inferred from the above, the major impediment to largeefficiencies of the present thermoelectric cells is the large amount ofradiation from the emitter. While this radiation increases rapidly withtemperature, the electrical power delivered to the load increases evenmore rapidly, so that etficiency is optimized at the maximum temperatureat which the emitter is structurally stable. It can also be shown thatfor any particular plasma resistance, the power deliverable to anexternal load is maximized when its value equals that of the plasmaresistance (treating both impedances as purely sensitive). A method offurther reducing losses appears to lie in interposing a series ofelectrically floating radiation shields between emitter and collector tore-radiate back to the emitter. This is at best a fringe benefit, assuch shields must be in the form of open mesh or grid construction topermit conduction through the plasma, thereby reducing the availableradiation surface.

Embodiments of the type illustrated in FIGURE 1 have been operated withefiiciencies as high as 13%, efficiency being defined as the ratio ofthe electrical power output to the sum of such output, the thermalenergy incident on the collector electrode and the thermal energy lostby conduction from the emitter to the adjacent metal structure. Suchefiiciencies were obtained using the 2 cm. tantalum emitter describedabove, a copper collector electrode spaced 0.1 cm. from the emitter, anda cesium vapor pressure of 2 mm. Hg. Efficiency was optimized at anemitter temperature of 2450 C., under which conditions the current drawnwas 60 amperes, the power delivered to an external resistor (R ofFIGURE 1) of 0.617 ohm was 60 watts, the measured radiant energyincident on the collector electrode was 500 watts, and the emitterconduction losses were calculated to be watts.

For applications such as the extraction of heat from a high power levelfission reactor, embodiments of the present invention are believed torepresent a stride forward in that at least a part of the conventionalcoolantturbine-generator equipment may be eliminated. The radiant heatin such applications is not thrown away, but is still available forexternal use. Optimization of design where such a reactor may be used tofurnish both heat and electrical energy may well eliminate suchconventional electrical power generation equipment in its entirety.

With the efficiencies of 13% for a tantalum emitter and with probablyeven greater efllciencies prevailing for the carbide emitters employinguranium carbide, it becomes possible to consider the plasma cells of thepresent invention as power sources for space vehicles. Thus the emittermay be heated by any number of the means mentioned above-using theemitter itself as the fissile fuel container, locating a criticalassembly outside the plasma cell or cells and utilizing the fissionenergy to heat the emitter, or using solar energy or the energy releasedby radioactive nuclides as heat sources.

The electrical energy thus made available by the plasma cells can beused both to ionize and heat a propellant gas such as hydrogen orhelium, andto energize coils to provide a magnetic field of highintensity. The ionized fuel will then be propelled through a nozzle ofthe vehicle by such magnetic fields and crossed electric fields'tofurnish the necessary thrust. The energy transferred to the magneticfield by the plasma cells will thus be continuously transferred to thepropellant.

What is claimed is:

1. Means for converting one form of energy into electrical energycomprising, in combination, a first electrically conductive refractorymaterial, a second electrically conductive material spaced from saidfirst electrically conductive refractory material, closure meansdefining a gas chamber between said electrically conductive refractorymaterials and a partial filling of an alkali metal in said chamber, saidfirst electrically conductive refractory material being at a temperatureof not less than 1500 K. during operation and comprising at least one ofthe carbides selected from the class consisting of uranium, zirconium,niobium, thorium, protactinium, plutonium, neptunium, and americium.

2. The device of claim 1 in which said first electrically conductingrefractory material includes about 80 mole percent zirconium carbideadmixed with uranium carbide in the form of a fused solid solution.

3. The device of claim 1 in which said carbides are fused on a backingof a refractory metal.

4. A device for extracting energy, including electrical energy, from Ucomprising in combination a critical assembly of fuel cells, each saidfuel cell comprising a core consisting of an electrically conductiverefractory combination of said U with uranium carbide diluted withzirconium carbide, an electrically conductive shell surrounding saidcore, electrically insulated therefrom, and defining a gap therewith anda partial filling of an alkali metal vapor in said gap, said gapotherwise containing only a hard vacuum, means for connecting each saidcore and each said shell to external electrical circuitry, and coolingmeans for extracting the heat energy of said shells andmaintaining saidcells at about the minimum temperature required for vaporizing saidalkali metal to a vapor particle density of 10 to 10 particles per cc.

5. A thermionic emitter structure, said structure comprising uraniumcarbide diluted with zirconium carbide, said carbides being solidifiedfrom a liquid solution.

6. The structure of claim 5 in which said carbides are coated on arefractory metal.

7. A device as in claim 4 wherein only a portion of said core contains Uthereby insuring a thermal graclient.

8. A nuclear reactor comprising a core, means for converting fissionheat generated in said core to electricity within said core, said meansincluding a plurality of thermionic fuel elements, each of said fuelelements including a fissionable cathode, an anode spaced from saidcathode, and an ionizable gas between said cathode and anode.

9. A nuclear reactor comprising a core, a neutron moderator in saidcore; means for converting fission heat generated in said core toelectricity within said core, said means including a plurality ofthermionic fuel elements, said fuel elements including a fissionablecathode, an anode, and an ionizable gas between said cathode and anode;and means for maintaining a temperature gradient between said cathodeand said anode.

10. A nuclear reactor comprising a core; means for converting fissionheat generated in said core to electricity within said core, said meansincluding a plurality of thermionic fuel elements, each of said fuelelements including a fissionable-material-containing cathode, an anodespaced from the cathode, and an ionizable gas in said space; meansincluding an electrical conductive fluid for cooling said fuel elements;means for electrically insulating said cathode and anode from saidcoolant; and means for removing fission gases produced 'by said cathodefrom said core.

11. A nuclear reactor comprising a core; means for converting fissionheat generated in said core to electricity within said core, said meansincluding a plurality of fuel elements having a plurality of diodes,each of said diodes having a fissionable-material-containing cathode andan anode, said fissionable materal being in sufficient quantity in saidcore to sustain a chain reaction, said diodes being series connected;means for passing an ionizable gas through said diodes for removingfission gases released from said cathodes and for neutralizing spacecharge in said diodes; and means for passing a heat transfer medium inheat-removing relationship with said anodes.

12. A nuclear reactor comprising a core; means for converting fissionheat generated in said core to electricity within said core, said meansincluding a plurality of fuel elements having a plurality of diodes,each of said diodes having a fissionable-material-containing cathode anda spaced anode; means for series connecting said diodes, said meansincluding support means for said cathodes; and means for removingfission gases released in said fuel elements and for neutralizing spacecharge in said diodes.

13. A thermionic fuel element comprising in combination a plurality ofdiodes each having a fissionable-material-containing cathode and ananode associated with each cathode and spaced therefrom; means forseries connecting said diodes; and an ionizable gas in said cathodeanodespacing.

14. The thermionic fuel element of claim 13 wherein said ionizable gasis sealed within said fuel element.

15. The thermionic fuel element of claim 13 including means for passingsaid ionizable gas through said fuel element.

16. A thermionic fuel element comprising in combination at least onefissionable-material-containing cathode, an anode spaced from saidcathode, means for removing heat from said anode, an ionizable gas insaid space; and means including said gas for removing fission productgases released by said cathode.

17. A thermionic fuel element comprising in combination a plurality offissionable-material-containing cathodes electrically insulated fromeach other, an anode associated with each cathode and spaced therefrom,each cathode and associated anode constituting a diode; means for seriesconnecting said diodes; means for maintaining said anode at atemperature lower than said cathode whereby electrons liberated at saidcathode move along a thermal gradient to the associated anode; anionizable gas in said cathode-anode spacing; and means for removingfission gases released from said cathodes.

18. The thermionic fuel elements of claim 17 including means forsupporting each of said cathodes in spaced relationship with theadjacent cathode and its associated anode, said means including anelectrical connection between said cathode and the anode of the adjacentdiode.

19. A thermionic fuel element comprising in combination a plurality ofdiodes having a cathode and anode, each of said cathodes containingfissionable material; insulating means between the cathodes of each ofsaid diodes and between the anodes of each of said diodes; means forsupporting said cathode of each of said diodes including an electricalconnection to the anode of the adjacent diode; and an ionizable gasbetween the cathode and the anode of each of said diodes.

20. The combination of claim 19 in which said insulating means includesa perforated insulator disk.

21. The combination of claim 19 including electrical insulationenclosing said fuel element and a sealed container enclosing saidelectrical insulation. 7

22. A thermionic fuel element comprising in combination a plurality offissionable-material-containing cathodes, an anode spaced from andassociated with each of said cathodes; means for series connecting saidplurality of cathodes and anodes, said means supporting said cathodes inspaced relationship; and an ionizable gas in said cathode-anode spacing.I

23. A thermionic fuel element comprising a container, electricalinsulation on the inside surface of said container, a plurality ofanodes supported in spaced relation adjacent said insulation, aplurality of insulating disks between said anodes and dividing thevolume of said container into a plurality of compartments, afissionablematerial-containing cathode supported in each of saidcompartments in spaced relation from said anode, an ionizible gasbetween each of said cathodes and its associated anode, and means forseries connecting said plurality of cathodes and anodes.

24-. A nuclear reactor comprising a core; means for directly convertingheat liberated in said core to electricity including a plurality ofthermionic fuel elements,

each of said fuel elements containing at least onefissionable-material-containing ctahode, an anode and an ionizable gas,at least one group of said thermionic fuel elements series connected;and load means connected to said at least one group.

25. A nuclear reactor comprising a core, means for directly converting aportion of the heat liberated in said core to electricity including aplurality of thermionic fuel elements containing at least onefissionable-material-containing cathode, an anode, and an ionizable gas;electrical means interconnecting said fuel elements with a load; meansfor maintaining said anode at a temperature lower than said cathode,said last-named means including a coolant passing through said core; andmeans exterior to said core for converting a portion of the heatcollected by said coolant in said core to electricity.

26. A thermoelectric converter comprising an anode, means for coolingthe anode, a cathode spaced therefrom, an easily ionized gas between theanode and cathode, means for ionizing said gas to form a plasmacomprising a source of fission fragments disposed adjacent said gas andmeans for establishing a field of neutron flux through said source tocause fissioning of said source.

27. A device for converting nuclear energy to electrical energycomprising a thermoelectric cell having a cathode containing a fissilematerial, the total fissile material within said device constituting acritical mass to sustain a chain nuclear fission reaction, a conductiveanode in spaced relationship with and electrically insulated from saidcathode, a gas between said anode and said cathode ionizable by fissionfragments from said cathode, means for cooling said anode, and means fortaking off the generated between said anode and said cathode.

28. In a nuclear reactor device containing a critical mass of fissilematerial to sustain a chain nuclear re-action, a thermoelectric cellcomprising a cathode containing a fissile material, the fissile materialof said cathode forming a part of the critical mass within the reactordevice, an electrically conductive anode in spaced relationship with andelectrically insulated from said cathode, a gas between said anode andsaid cathode ionizable by fission fragments from said cathode, means forcooling said anode, and means for taking off the generated between saidanode and said cathode.

29. A thermionic emitter comprising uranium carbide and zirconiumcarbide, said carbides being in approximately the ratio of 80 atomicpercent ZrC and 20 atomic percent UC.

30. The thermionic emitter of claim 29 wherein the carbides are in theform of a solid solution.

31. A thermionic fuel element for a nuclear reactor, said fuel elementincluding a fissionable cathode, an anode spaced from said cathode, andan ionizable gas between said cathode and anode.

32. A thermionic fuel element for a nuclear reactor, said fuel elementincluding a fissionable cathode, an anode, and an ionizable gas betweensaid cathode and anode; and means for maintainin-g a temperaturegradient between said cathode and said anode.

33. A thermionic fuel element for a nuclear reactor, said fuel elementincluding a fissionable-material-containing cathode, an anode spacedfrom the cathode, and an ionizable gas in said space; means including anelectrically conductive fluid for cooling said fuel elements; means forelectrically insulating said cathode and anode from said coolant; andmeans for removing fission gases produced by said cathode from saidcore.

34. A thermionic fuel element for a nuclear reactor, said fuel elementhaving a plurality of diodes, each of said diodes having afissionable-material-containing cathode and an anode, said fissionablematerial being in sufficient quantity in said core to sustain a chainreaction, said diodes being series connected; means for passing anionizable gas through said diodes for removing fission gases releasedfrom said cathodes and for neutralizing space charge in said diodes; andmeans for passing a heat transfer medium in heat-removing relationshipwith said anodes.

35. A thermionic fuel element for a nuclear reactor, said fuel elementhaving a plurality of diodes, each of said diodes having afissionable-material-containing cathode and a spaced anode; means forseries connecting said diodes, said means including support means forsaid cathodes; and means for removing fission gases released in saidfuel elements and for neutralizing space charge in said diodes.

36. A thermionic fuel element for a nuclear reactor comprising a cathodecontaining a fissile material, a conductive anode in spaced relationshipwith and electrically insulated from said cathode, a gas between saidanode and said cathode ionizable by fission fragments from said cathode,means for cooling said anode, and means for taking off the generatedbetween said anode and said cathode.

37. A thermionic fuel element for a nuclear reactor comprising a cathodecontaining a fissile material, a conductive anode in spaced relationshipwith and electrically insulated from said cathode, means for circulatingbetween said anode and said cathode a gas ionizable by fission fragmentsfrom said cathode, and means for taking off the generated between saidanode and said cathode.

38. A thermionic fuel element for a nuclear reactor comprising a cathodecontaining a fissile material, and electrically conductive anode inspaced relationship with and electrically insulated from said cathode, agas between said anode and said cathode ionizable by fission fragmentsfrom said cathode, means for cooling said anode, and means for taking011 the generated between said anode and said cathode.

References Cited by the Examiner UNITED STATES PATENTS 2,661,431 12/1953Linder 310-4 X 2,980,819 4/1961 Feaster 313-212 3,005,766 10/1961Bartnotf 176-39 3,008,890 11/1961 Bartnofir 176-39 3,041,260 6/1962Goeddel 176-89 X 3,087,877 4/1963 Goeddel 176-89 X 3,093,567 6/ 196 3Jablonski et al. 176-52 3,113,091 12/1963 Rasor et al 176-30 REUBENEPSTEIN, Primary Examiner. CARL D. QUARFORTH, Examiner.

1. MEANS FOR CONVERTING ONE FORM OF ENERGY INTO ELECTRICAL ENERGYCOMPRISING, IN COMBINATION, A FIRST ELECTRICALLY CONDUCTIVE REFRACTORYMATERIAL, A SECOND ELECTRICALLY CONDUCTIVE MATERIAL SPACED FROM SAIDFIRST ELECTRICALLY CONDUCTIVE REFRACTORY MATERIAL, CLOSURE MEANSDEFINING A GAS CHAMBER BETWEEN SAID ELECTRICALLY CONDUCTIVE REFRACTORYMATERIALS AND A PARTIAL FILLING OF AN ALKALI METAL IN SAID CHAMBER, SAIDFIRST ELECTRICALLY CONDUCTIVE REFRACTORY MATERIAL BEING AT A TEMPERATUREOF NOT LESS THAN 1500*K. DURING OPERATION AND COMPRISING AT LEAST ONE OFTHE CAR-